The top image shows the initial state of a planet-forming disk of gas and dust. The disk is viewed at an angle of roughly 45$rm o$ from the top. The yellow dot is the Sun. The concentration of gas and dust in the disk decreases away from the Sun blue represents the largest concentration of gas and dust green, a medium concentration and red, the lowest concentration. Planetesimals grow first in the inner portions of the disk, where the concentration of gas and dust is largest. Many thousands of collisions among the largest planetesimals produce planets as large as Pluto. As the largest planetesimals grow into planets, they stir up the smaller planetesimals. Millions of collisions among the smaller planetesimals produce billions of tiny dust grains, which reflect the light of the Sun. By the time a planet grows larger than Pluto, the dust grains along its orbit produce a bright ring observable from the Earth (second panel from the top). The planet then begins to sweep up the dust, clearing a path along its orbit. By the time this planet has cleared the dust from its orbit, another planet and its bright ring of dust have formed farther out in the disk (third panel from the top). In panels 4, 5, and 6, the process of dust clearing and planet formation moves out through the disk, until a few planets have cleared almost all of the dust of out the disk. In a disk around a Sun-like star, this entire process takes more than 100 Myr.